A 168 dB FoM ISFET-Integrated CT Delta Sigma Frontend With Gm-Boosted Linearity and Hybrid Noise Shaping Achieving 6.8 m-pH Resolution

This paper presents a low-power, ISFET-integrated frontend architecture that directly merges the sensing element with a Gm-C based continuous-time delta sigma modulator (CT-ΔΣM). In the proposed design, the ISFET simultaneously functions as both the biochemical sensor and the input common-source stage of the integrator, eliminating the need for intermediate driver and thereby improving energy efficiency. A passive low-pass filter DAC (LPF-DAC) is introduced to attenuate the input-feedback residue and provide hybrid noise shaping. The input Gm combines source degeneration and gm-boost to mitigate the nonlinearities introduced by input-feedback residue and common-mode variation. Fabricated in 180 nm CMOS, the proposed CT-ΔΣM achieves a measured peak SNDR of 84.2 dB and a dynamic range of 90.4 dB over a 10 kHz bandwidth under a 900 mVpp input, with a power consumption of only 41.5 µW. These results present a Schreier FoM of 168 dB, representing a 8 dB improvement over prior ISFET frontends. The frontend demonstrates an averaged sensitivity of 29.04 mV/pH, along with a resolution of 6.8 m-pH, validating its applicability for lowpower, high-accuracy biochemical sensing.